Light emitting device having transparent electrode and method of manufacturing light emitting device

ABSTRACT

Provided are a light emitting device including a transparent electrode having high transmittance with respect to light in a UV wavelength range as well as in a visible wavelength range and good ohmic contact characteristic with respect to a semiconductor layer and and a method of manufacturing the light emitting device. A transparent electrode of a light emitting device is formed by using a resistance change material which has high transmittance with respect to light in a UV wavelength range and of which resistance state is to be changed from a high resistance state into a low resistance state due to conducting filaments, which current can flow through, formed in the material if a voltage exceeding a threshold voltage inherent in a material applied to the material, so that it is possible to obtain high transmittance with respect to light in a UV wavelength range.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 14/413,911, filed on Jan. 9, 2015 (currently pending), thedisclosure of which is herein incorporated by reference in its entirety.The U.S. patent application Ser. No. 14/413,911 is a national entry ofInternational Application No. PCT/KR2012/007256, filed on Sep. 10, 2012,which claims priority to Korean Application No. 10-2012-0075651 filed onJul. 11, 2012, the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light emitting device including atransparent electrode and a method of manufacturing the light emittingdevice, and more particularly, to a light emitting device including atransparent electrode having good ohmic contact characteristic and hightransmittance and a method of manufacturing the light emitting device.

2. Description of the Related Art

Transparent electrodes have been used in various application fields suchas LEDs, solar cells, medical UV sterilizers, and fisheries, and theapplication fields and their demands have been gradually increased.Particularly, the transparent electrodes have been actively used in theLED field. The transparent electrode technique currently applied to theLEDs is mainly an ITO (indium tin oxide) based technique which can beapplied to a visible wavelength range of 400 nm to 800 nm and a UVwavelength range of 365 nm to 400 nm in the entire UV wavelength rangeof 10 nm to 400 nm.

Recently, demands for UV LEDs generating light in a UV wavelength rangehas been greatly increased. However, a transparent electrode having highconductivity and high transmittance in the UV wavelength range has notbeen developed, so that it is difficult to commercialize the UV LEDs.

For example, in the case of a UV LED where an ITO transparent electrodewhich is currently actively used is formed, most of light in a UVwavelength range of 10 nm to 320 nm generated in an activation layer isabsorbed by an ITO layer, so that only about 1% of the light can betransmitted through the ITO layer to be extracted to an externalportion.

FIG. 1 is a graph illustrating transmittance in the case where an ITOtransparent electrode is formed on a p-GaN semiconductor layer in therelated art. As illustrated in FIG. 1, although the transparentelectrode has transmittance of 80% or more with respect to the light ina wavelength range of 350 nm or more, the transmittance is greatlydecreased with respect to the light having a short wavelength in the UVwavelength range. Particularly, the transmittance is decreased to 20% orless with respect to the light having a short wavelength in the UVwavelength range of 280 nm or less.

In order to solve the above problem, in the related art, a metalelectrode pad is directly formed on a semiconductor layer such asp-AlGaN instead of forming the transparent electrode on thesemiconductor layer. However, the metal and the semiconductor layer arenot in ohmic contact to each other because of a large difference in workfunction between the metal and the semiconductor layer, and current isconcentrated on a metal electrode pad, but current is not supplied intothe entire activation layer, so that an amount of the light generated bythe activation layer is greatly decreased.

In order to solve the above problem, various researches have been made,but a transparent electrode having high conductivity and hightransmittance in a UV wavelength range has not yet been developed. Thisis because conductivity and transmittance of a material is basically intrade-off relationship. Since a material having high transmittance in aUV wavelength range has a large band gap, the conductivity thereof istoo low to be used as an electrode, and since the material is not inohmic contact with a semiconductor material, it is impossible to use theabove material as an electrode.

As an example of a technique for solving the above problem, a techniquewhere a transparent electrode is constructed with a sliver (Ag) thinfilm is disclosed in Korean Patent Application No. 10-2007-0097545.However, in the related art, in the case where the transparent electrodeis formed by using Ag, it is very difficult to deposit a thin silverlayer on a semiconductor layer so that the thin sliver layer is in ohmiccontact with the semiconductor layer. In addition, although a thinsilver layer is deposited on the semiconductor layer, as illustrated inthe graph of FIG. 4 of the above Patent Document, with respect to thelight in a wavelength range 420 nm or less, the transmittance is greatlydecreased to 80% or less; and with respect to the light in a wavelengthrange 380 nm or less, the transmittance is decreased to 50% or less.Therefore, the transmittance in the above-described technique has nodifference from the transmittance of the ITO transparent electrode inthe related art, and thus, it is difficult to improve the transmittancein a UV wavelength range up to a practical level.

SUMMARY OF THE INVENTION

The present invention is to provide a light emitting device including atransparent electrode having high transmittance with respect to light ina UV wavelength range as well as in a visible wavelength range and goodohmic contact characteristic with respect to a semiconductor layer and amethod of manufacturing the light emitting device.

According to an aspect of the present invention, there is provided alight emitting device including: a substrate; a first semiconductorlayer which is formed on the substrate; an activation layer which isformed on the first semiconductor layer to generate light; a secondsemiconductor layer which is formed on the activation layer; and antransparent electrode which is formed on the second semiconductor layerby using a transparent insulating material of which resistance state ischanged from a high resistance state into a low resistance stateaccording to an applied electric field.

In the above aspect, the light emitting device may further include areflective layer formed on the transparent electrode.

In addition, in the above aspect, the substrate and a submount substratemay be combined to each other so that the reflective layer is in contactwith a first conductive pattern formed on the submount substrate and anelectrode pad formed on an externally exposed portion of the firstsemiconductor layer is in contact with a second conductive patternformed on the submount substrate through a bump.

According to another aspect of the present invention, there is provideda light emitting device including: a substrate; a reflective layer whichis formed on the substrate; a transparent electrode which is formed onthe reflective layer by using a transparent insulating material of whichresistance state is changed from a high resistance state into a lowresistance state according to an applied electric field; a secondsemiconductor layer which is formed on the transparent electrode; anactivation layer which is formed on the second semiconductor layer togenerate light; and a first semiconductor layer which is formed on theactivation layer.

In the above aspect, a forming process may be performed on thetransparent electrode by applying a threshold voltage or more inherentin a material of the transparent electrode, so that conducting filamentsare formed in the transparent electrode.

In addition, in the above aspect, the first semiconductor layer may beformed with an n-AlGaN layer, and the second semiconductor layer isformed with a p-AlGaN layer or a p-AlGaN layer and a p-GaN thin film.

In addition, in the above aspect, the transparent electrode may be inohmic contact with the second semiconductor layer.

In addition, in the above aspect, the transparent electrode may beformed with any one of a transparent oxide based material, a transparentnitride based material, a transparent polymer based material, and atransparent nano material.

In addition, in the above aspect, the light emitting device may furtherinclude a current spreading layer which is formed with CNT (carbon nanotube) or graphene between the second semiconductor layer and thetransparent electrode.

In addition, in the above aspect, the light emitting device may furtherinclude a current spreading layer which is formed with CNT or grapheneto be in contact with a surface of the transparent electrode opposite toa surface of the transparent electrode which is in contact with thesecond semiconductor layer.

According to still another aspect of the present invention, there isprovided a method of manufacturing a light emitting device, including:(a) sequentially forming a first semiconductor layer, an activationlayer which generates light, and a second semiconductor layer on asubstrate; (b) forming a transparent electrode on the secondsemiconductor layer by using a transparent insulating material of whichresistance state is to be changed from a high resistance state into alow resistance state according to an applied electric field; and (c)changing the resistance state of the transparent electrode into the lowresistance state by applying a voltage to the transparent electrode.

In addition, in the above aspect, the method may further include (d)forming a reflective layer on the transparent electrode.

In addition, in the above aspect, the method may further include: (e)etching the transparent electrode, the second semiconductor layer, andthe activation layer so that the first semiconductor layer is exposedand forming an electrode pad on the first semiconductor layer; and (f)combining the substrate with a submount substrate so that the reflectivelayer is in contact with a first conductive pattern formed on thesubmount substrate and the electrode pad is in contact with the secondconductive pattern formed on the submount substrate through a bump.

In addition, in the above aspect, the method may further include: (e)forming a bonding layer on the reflective layer and attaching a submountsubstrate to the bonding layer; and (f) separating the substrate fromthe first semiconductor layer.

In addition, in the above aspect, the (c) changing of the resistancestate may be performing a forming process by applying a thresholdvoltage or more to the transparent electrode, so that conductingfilaments are formed in the transparent electrode.

In addition, in the above aspect, the first semiconductor layer may beformed with an n-AlGaN layer, and the second semiconductor layer isformed with a p-AlGaN layer or a p-AlGaN layer and a p-GaN thin film.

In addition, in the above aspect, the transparent electrode may be inohmic contact with the second semiconductor layer.

In addition, in the above aspect, the transparent electrode may beformed with any one of a transparent oxide based material, a transparentnitride based material, a transparent polymer based material, and atransparent nano material.

In addition, in the above aspect, the method may further include,between the (a) sequentially forming of the first semiconductor layer,the activation layer which generates light, and the second semiconductorlayer and the (b) forming of the transparent electrode, forming acurrent spreading layer on the second semiconductor layer by using CNTor graphene, wherein, in the (b) forming of the transparent electrode,the transparent electrode is formed on the current spreading layer.

In addition, in the above aspect, the method may further include forminga current spreading layer on the transparent electrode of whichresistance state is changed into the low resistance state by using CNTor graphene.

According to the present invention, a transparent electrode of a lightemitting device is formed by using a resistance change material whichhas high transmittance with respect to light in a UV wavelength rangeand of which resistance state is to be changed from a high resistancestate into a low resistance state due to conducting filaments, whichcurrent can flow through, formed in the material if a voltage exceedinga threshold voltage inherent in a material applied to the material, sothat it is possible to obtain high transmittance with respect to lightin a UV wavelength range (particularly, light in a wavelength range of340 nm to 280 nm and light in a wavelength range of 280 nm or less) aswell as in a visible wavelength range generated by the light emittingdevice and to obtain good ohmic contact characteristic with respect to asemiconductor layer due to high conductivity of the transparentelectrode.

In addition, according to the present invention, a current spreadinglayer formed by using CNT or graphene having good ohmic contactcharacteristic and high transmittance is formed on an upper or lowerportion of a transparent electrode to connect conducting filamentsformed in the transparent electrode, so that current flowing into thetransparent electrode is allowed to spread through the entiresemiconductor layer, and thus, it is possible to prevent the problem ofcurrent concentration from occurring.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent by describing in detail exemplary embodimentsthereof with reference to the attached drawings in which:

FIG. 1 is a diagram illustrating transmittance in the case where an ITOtransparent electrode is formed on a p-GaN semiconductor layer in therelated art;

FIG. 2 is a diagram illustrating a configuration of a light emittingdevice including a transparent electrode according to a first embodimentof the present invention;

FIGS. 3A and 3B are diagrams illustrating characteristics of aresistance change material;

FIG. 4 is a diagram illustrating a method of manufacturing the lightemitting device according to the first embodiment of the presentinvention;

FIGS. 5A and 5B are diagrams illustrating a configuration of a lightemitting device according to a modified example of the first embodimentof the present invention in order to solve a problem of currentconcentration;

FIG. 6A is a diagram illustrating a configuration of a light emittingdevice according to a second embodiment of the present invention;

FIG. 6B is a diagram illustrating a modified example of the secondembodiment;

FIG. 7 is a diagram illustrating a method of manufacturing a lightemitting device according to the second embodiment of the presentinvention.

FIG. 8A is a diagram illustrating a configuration of a light emittingdevice according to a third embodiment of the present invention;

FIG. 8B is a diagram illustrating a configuration of a light emittingdevice according to a modified example of the third embodiment;

FIG. 9 is a diagram illustrating a method of manufacturing the lightemitting device according to the third embodiment of the presentinvention; and

FIGS. 10A to 10E are graphs illustrating a transmittance characteristic,an ohmic characteristic before the forming process, a contact resistancecharacteristic before the forming process, an ohmic characteristic afterthe forming process, and a contact resistance characteristic after theforming process in the case where a transparent electrode is formed on ap-GaN semiconductor layer by using a Ga₂O₃ material.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, exemplary embodiments of the present invention will bedescribed with reference to the attached drawings.

FIG. 2 is a diagram illustrating a configuration of a light emittingdevice including a transparent electrode 260 according to a firstembodiment of the present invention.

Referring to FIG. 2, the light emitting device according to the firstembodiment of the present invention is configured to include, asubstrate 210 and a buffer layer 220, a first semiconductor layer 230,an activation layer 240, and a second semiconductor layer 250 which aresequentially formed on the substrate 210. In addition, the lightemitting device is configured to include a transparent electrode 260formed on the second semiconductor layer 250, an electrode pad 270 aformed on an upper portion of the transparent electrode 260, and anelectrode pad 270 b formed on an upper portion of the firstsemiconductor layer 230 of which a portion region is etched to beexposed.

The substrate 210 may be constructed by using a substrate such as asapphire substrate which is generally used for manufacturing a lightemitting device. The buffer layer 220 may be formed with un-doped GaN orthe like so as to allow the first semiconductor layer 230 to be easilygrown, and if needed, the buffer layer 220 may be omitted.

The first semiconductor layer 230 is a semiconductor layer doped as an ntype. In the embodiment of the present invention, the firstsemiconductor layer 230 is formed with n-AlGaN so as to generate lightin a UV wavelength range. However, the first semiconductor layer 230 mayalso be formed with a general material used for manufacturing a lightemitting device capable of generating light in a UV wavelength range.

The activation layer 240 (MQW) is preferably formed withAl(In)GaN/(In)GaN capable of generating light in a UV wavelength range.However, any material capable of generating light in a UV wavelengthrange can be used without limitation.

The second semiconductor layer 250 is a semiconductor layer doped as a ptype. In the embodiment of the present invention, the secondsemiconductor layer 250 is formed with a single layer of p-AlGaN so asto generate light in a UV wavelength range, or the second semiconductorlayer 250 is formed by sequentially forming a p-AlGaN layer and a p-GaNthin film on the activation layer 240. However, the second semiconductorlayer 250 may also be formed with a general material used formanufacturing a light emitting device capable of generating light in aUV wavelength range.

In the above-described embodiment, the first semiconductor layer 230 andthe second semiconductor layer 250 are semiconductor layers doped as ann type and a p type, respectively. The reverse case is also available.

The transparent electrode 260 according to the present invention isconstructed with a transparent material (resistance change material)which has high transmittance with respect to light including light in aUV wavelength range and of which resistance state is to be changedaccording to an applied electric field. The resistance change materialis mainly used in the field of ReRAM (resistive RAM). If a thresholdvoltage or more inherent in the material is applied to the material,electro-forming is performed, the resistance state of the resistancechange material which is originally an insulating material is to bechanged from a high resistance state into a low resistance state, sothat the material has conductivity.

More specifically, if a threshold voltage or more is applied to theresistance change material which is an insulating material, electrodemetal materials are diffused into a thin film due to electric stress(i.e., forming process), or a defective structure occurs in the thinfilm, so that conducting filaments 262 (or, metallic filaments) areformed in the resistance change material as illustrated in FIG. 2. Afterthat, although the voltage applied to the material is removed, theconducting filaments 262 remain, and current can flow through theconducting filaments 262, so that the low resistance state of thematerial is maintained.

Referring to FIG. 3A, it can be seen that the resistance change material(for example, AlN) has an insulating characteristic before the formingprocess and has an I-V characteristic of a metal after the formingprocess.

FIG. 3B is a graph illustrating how long the resistance state can bestably maintained after the conducting filaments 262 are formed. It canbe seen from a dotted line in the graph that the low resistance statecan be stably maintained for ten years after the conducting filaments262 are formed.

In the embodiment of the present invention, a transparent conductiveoxide based material (SiO₂, Ga₂O₃, Al₂O₃, ZnO, ITO, or the like), atransparent conductive nitride based material (Si₃N₄, AlN, GaN, InN, orthe like), a transparent conductive polymer based material(polyaniline(PANI)), poly(ethylenedioxythiophene)-polystyrene sulfonate(PEDOT: PSS) or the like), and a transparent conductive nano material(CNT, CNT-oxide, Graphene, Graphene-oxide, or the like) or the like maybe used as the resistance change material. In addition to theabove-described materials, any material which is transparent and has theabove-described resistance change characteristic can be used to form thetransparent electrode 260 according to the present invention. It shouldbe noted that the statement that the material has conductivity denotesthat the material is allowed to have conductivity as a result of theforming process by which the conducting filaments 262 are formed in thetransparent electrode. In addition, it should be noted that the formingprocess is performed on the transparent electrode 260 according to thepresent invention, so that the conducting filaments are formed in thetransparent electrode.

As illustrated in FIG. 2, if the light emitting device is completed,current injected through the electrode pads 270 a formed on thetransparent electrode 260 is allowed to spread through the conductingfilaments 262, which are connected to each other in the transparentelectrode 260, over the entire area to be injected into the entiresecond semiconductor layer 250. The light generated in the activationlayer 240, particularly, the light in a UV wavelength range is emittedthrough the transparent electrode 260 having a large band gap to anexternal portion.

FIG. 4 is a diagram illustrating a method of manufacturing the lightemitting device according to the first embodiment of the presentinvention.

The method of manufacturing the light emitting device according to thefirst embodiment of the present invention will be described withreference to FIG. 4. First, by using the same method as a method ofmanufacturing a light emitting device in the related art, a buffer layer220, a first semiconductor layer 230, an activation layer 240, and asecond semiconductor layer 250 is formed on a substrate 210. Next, byusing the same method as a general method of fabricating a transparentelectrode, a transparent electrode 260 is formed with a resistancechange material described above on the second semiconductor layer 250(refer to (a) of FIG. 4).

Next, a photoresist layer 280 is formed on the transparent electrode260, and by performing a photolithography process, a pattern for forminga forming electrode 282 is formed on a portion of a region where a metalpad 270 a is to be formed in the photoresist layer 280 (refer to (b) ofFIG. 4). Next, by performing an e-beam process, a sputtering process, orother metal deposition processes, the forming electrode 282 is formed inthe pattern. Next, the forming electrode 282 is completed by removingthe photoresist layer 280 except for the forming electrode 282 through alift-off process.

Next, as illustrated in (d) of FIG. 4, if a threshold voltage or moreinherent in the material is applied to the forming electrode 282 formedon the transparent electrode 260, conducting filaments 262 are formed inthe transparent electrode 260 which is an insulating material, so thatthe resistance state of the transparent electrode 260 is changed from ahigh resistance state into a low resistance state.

After the conducting filaments 262 are formed in the transparentelectrode 260, a metal electrode pad 270 a is formed on the transparentelectrode 260 (refer to (e) of FIG. 4). At this time, as a method offorming the metal electrode pads 270 a, the forming electrode 282 forperforming the forming process may be removed, and a separate metalelectrode pad 270 a may be formed. Alternatively, as illustrated in (e)of FIG. 4, metal is additionally deposited on the forming electrode 282by using a mask 284, so that the metal electrode pad 270 a may beformed.

Next, by using the same method as a general method of manufacturing ahorizontal type light emitting device, the second semiconductor layer250 and the activation layer 240 are sequentially etched from thetransparent electrode 260 so as to allow the first semiconductor layer230 to be exposed, and the n-type electrode pad 270 b is formed on thefirst semiconductor layer 230 (refer to (f) of FIG. 4).

Hereinbefore, the light emitting device according to the firstembodiment of the present invention and the method of manufacturing thelight emitting device are described.

In the first embodiment described above with reference to FIGS. 2 to 4,some conducting filaments 262 formed in the transparent electrode 260may not be connected to other conducting filaments 262. In this case,current flowing into the transparent electrode 260 may not spread overthe entire transparent electrode 260 but be concentrated to belocalized, so that a problem of current concentration that current isconcentrated to be localized on the second semiconductor layer 250 whichis in contact with the transparent electrode 260 may occur.

FIGS. 5A and 5B are diagrams illustrating configurations of lightemitting devices according to modified examples of the first embodimentof the present invention for solving the problem of currentconcentration.

In the examples illustrated in FIGS. 5A and 5B, in order to improve thecurrent spreading characteristic of the transparent electrode 260, acurrent spreading layer 290 formed by using CNT (carbon nano tube) orgraphene which connects the conducting filaments 262 formed on thetransparent electrode 260 is formed on an upper surface or a lowersurface of the transparent electrode 260. FIG. 5A illustrates an examplewhere the current spreading layer 290 formed by using CNT or graphene isformed on the transparent electrode 260. FIG. 5B illustrates an examplewhere the current spreading layer 290 formed by using CNT or graphene isformed between the transparent electrode 260 and the secondsemiconductor layer 250.

The CNT and the graphene have good conductivity and transmittancecharacteristics. In the present invention, the conducting filaments 262in the transparent electrode 260 are connected to each other by formingthe current spreading layer 290 with CNT or graphene on one surface ofthe transparent electrode 260 by using the characteristics, so that thecurrent flowing into the transparent electrode 260 can be allowed tospread over the entire second semiconductor layer 250.

At this time, as the thickness of the current spreading layer 290 isincreased, the CNTs or graphenes in the current spreading layer 290 areconnected to each other, and thus, the possibility that the conductingfilaments 262 are connected to each other is increased. As a result, theconductivity of the transparent electrode 260 is increased, but thetransmittance thereof is decreased. Therefore, it is preferable that thecurrent spreading layer 290 according to the present invention is formedwith thinness enough to connect the conducting filaments 262 in thetransparent electrode 260 to each other and as thin as possible within arange where the transmittance is not deteriorated.

In the embodiment of the present invention illustrated in FIGS. 5A and5B, the current spreading layer 290 is formed with a thickness of about2 nm to about 100 nm. The thickness of 2 nm is a minimum thickness sothat a single layer of CNT or graphene can be formed, and the thicknessof 100 nm is a maximum thickness so that transmittance can be maintainedto be 80% or more.

The configurations of the examples illustrated in FIGS. 5A and 5B arethe same as those of the example described with reference to FIGS. 2 to4, except that the current spreading layer 290 is formed with CNT orgraphene just before or after the transparent electrode 260 is formed,and thus, the detailed description thereof is omitted.

FIG. 6A is a diagram illustrating a configuration of a light emittingdevice according to a second embodiment of the present invention. FIG.6B is a diagram illustrating a modified example of the secondembodiment;

First, referring to FIG. 6A, the light emitting device according to thesecond embodiment of the present invention has a flip-chip structurewhere light generated in an activation layer is emitted in the directionof a substrate. The light emitting device is configured by furtherincluding a transparent electrode formed by using a resistance changematerial of the present invention, in which conducting filaments areformed, between a semiconductor layer (p-GaN) and a reflective layer inaddition to the light emitting device having a flip-chip structure inthe related art.

In the second embodiment illustrated in FIG. 6A, the surface where asubstrate 610 is disposed is referred to as a lower surface. In the samemanner as that of the horizontal type light emitting device describedwith reference to FIGS. 2 to 4, a substrate 610, a buffer layer 620, afirst semiconductor layer 630, an activation layer 640, and a secondsemiconductor layer 650 are sequentially formed, a transparent electrode660 where conducting filaments 662 are formed by performing a formingprocess is formed on the second semiconductor layer 650, and areflective layer 670 is formed on the transparent electrode 660. Herein,the above-described components of the substrate 610 to the transparentelectrode 660 may be formed by using the same material and manner asthose of the components of the substrate 210 to the transparentelectrode 260 illustrated in FIG. 2, and the reflective layer 670 may beformed by using Ag, Al, or the like which is used for a light emittingdevice having a general flip-chip structure.

After the reflective layer 670 is formed, the reflective layer 670 isconnected to a first conductive pattern 680 a formed on a submountsubstrate 690, and an electrode pad 672 formed on the firstsemiconductor layer 630 is connected to a second conductive pattern 680b formed on the submount substrate 690 by a bump 674, so that thecurrent can be supplied.

The current injected through the first conductive pattern 680 a isapplied to the transparent electrode 660 through the reflective layer670 and is allowed to spread through conducting filaments 662 formed inthe transparent electrode 660 over the entire second semiconductor layer650.

The light generated in the activation layer 640 is directed to the upperside where the first semiconductor layer 630 is disposed and to thelower side where the second semiconductor layer 650 is disposed. Thelight directed to the lower side passes through the transparentelectrode 660 and is reflected by the reflective layer 670 to bedirected to the upper side. Subsequently, the light passes through thesemiconductor substrate 610 to be emitted to an external portion.

At this time, since the transparent electrode according to the presentinvention 660 is in a low resistance state due to the conductingfilaments 662, the transparent electrode has a good ohmic contactcharacteristic with respect to the second semiconductor layer 650 andthe reflective layer 660. In addition, since the transparent electrodeis constructed with a material having a large band gap and having hightransmittance with respect to the light in a UV wavelength range, thetransparent electrode has high transmittance.

FIG. 7 is a diagram illustrating a method of manufacturing a lightemitting device according to the second embodiment of the presentinvention.

The method of manufacturing the light emitting device according to asecond embodiment of the present invention will be described withreference to FIG. 7. The transparent electrode 660 where the conductingfilaments 662 are formed by performing the above-described steps of (a)to (d) of FIG. 4 is formed on the second semiconductor layer 650 (referto (a) of FIG. 7). After the forming electrode is removed, thereflective layer 670 is formed on the second semiconductor layer 650(refer to (b) of FIG. 7). At this time, the reflective layer 670 may beformed over the entire transparent electrode 660. Alternatively, thereflective layer 670 may also be formed only the area of the firstsemiconductor layer 630 by using the metal mask 710 except for the areawhich is to be etched in order to form the electrode pad 672 on thefirst semiconductor layer 630.

Next, in order to form the electrode pad 672 on the first semiconductorlayer 630, a predetermined area of the light emitting device is etchedso as to expose the first semiconductor layer 630 from the transparentelectrode 660 (or the reflective layer 670), and the electrode pad 672is formed on the first semiconductor layer 630 (refer to (c) of FIG. 7).

Next, the submount substrate 690 where the first conductive pattern 680a the second conductive pattern 680 b are formed is prepared, thereflective layer 670 is bonded to the first conductive pattern 680 a,and the second conductive pattern 680 b and the electrode pad 672 formedon the first semiconductor layer 630 are bonded to each other through abump 674. The light emitting device is faced down and the submountsubstrate 690 is bonded, so that the light emitting device having aflip-chip structure is completed (refer to (d) of FIG. 7).

FIG. 6B is a diagram illustrating a modified example of the secondembodiment of the present invention. In the case where a portion of theconducting filaments 662 formed in the transparent electrode 660 is notconnected to other conducting filaments 662, in order to prevent currentfrom being concentrated on the second semiconductor layer 650 throughonly the portion of the conducting filaments 662, the current spreadinglayer 690 formed by using CNT or graphene is additionally formed betweenthe second semiconductor layer 650 and the transparent electrode 660.Although the current spreading layer 690 is additionally formed betweenthe second semiconductor layer 650 and the transparent electrode 660 inFIG. 6B, the current spreading layer 690 may be additionally formedbetween the transparent electrode 660 and the reflective layer 670.

FIG. 8A is a diagram illustrating a configuration of a light emittingdevice according to a third embodiment of the present invention. FIG. 8Bis a diagram illustrating a configuration of a light emitting deviceaccording to a modified example of the third embodiment.

The light emitting device according to the third embodiment of thepresent invention is a vertical type light emitting device. Thetransparent electrode formed by using a resistance change material inwhich the conducting filaments are formed is additionally formed betweenthe reflective layer and the second semiconductor layer (for example,p-GaN layer) in a configuration of a general vertical type lightemitting device.

Referring to FIG. 8A, the light emitting device according to the thirdembodiment of the present invention is configured by sequentiallyforming a submount substrate 890, an bonding layer 880, a reflectivelayer 870, a transparent electrode 860, a second semiconductor layer850, an activation layer 840, a first semiconductor layer 830, and anelectrode pad 910.

In the third embodiment, the submount substrate 890 is constructed witha metal substrate into which current can be injected. The reflectivelayer 870 is formed on the metal substrate 810 by using a material suchas Al or Ag which is generally used for the reflective layer 870 of thelight emitting device. The reflective layer 870 reflects the lightgenerated in the activation layer 840 to direct the light toward theupper side.

Similarly to the first and second embodiments, the transparent electrode860 formed on the reflective layer is formed by using a transparentresistance change material which can pass the light in a UV wavelengthrange and of which resistance state is to be changed from a highresistance state into a low resistance state due to the conductingfilaments formed in the material if a threshold voltage or more inherentin the material is applied to the material. In addition, similarly tothe first and second embodiments, the conducting filaments 862 areformed in the transparent electrode due to the forming process, so thatthe low resistance state of the transparent electrode is maintained; andthe current applied from the reflective layer 870 is allowed to spreadover the entire area through the conducting filaments 862 formed in thetransparent electrode 860, so that the current is injected into thesecond semiconductor layer 850.

The second semiconductor layer 850 is a semiconductor layer doped as a ptype. In the embodiment of the present invention, the secondsemiconductor layer 850 is formed by using a single layer of p-AlGaN soas to generate light in a UV wavelength range. Alternatively, the secondsemiconductor layer 850 may be formed by sequentially forming a p-GaNthin film and a p-AlGaN layer on the transparent electrode. However, thesecond semiconductor layer 850 may be formed by using a general materialused for manufacturing a light emitting device capable of generatinglight in a UV wavelength range.

The activation layer 840 (MQW) is preferably formed by usingAl(In)GaN/(In)GaN so as to generate light in a UV wavelength range.However, any material capable of generating light in a UV wavelengthrange can be used without limitation.

The first semiconductor layer 830 is a semiconductor layer doped as an ntype. In the embodiment of the present invention, the firstsemiconductor layer 830 is formed by using n-AlGaN so as to generatelight in a UV wavelength range. However, the first semiconductor layer830 may also be formed by using a general material used formanufacturing a light emitting device capable of generating light in aUV wavelength range.

In the above-described embodiment, the first semiconductor layer 830 andthe second semiconductor layer 850 are semiconductor layers doped as ann type and a p type, respectively. The reverse case is also available.

FIG. 9 is a diagram illustrating a method of manufacturing the lightemitting device according to the third embodiment of the presentinvention.

Referring to FIG. 9, by performing the same processes as Steps (a) to(d) of FIG. 4, the buffer layer 820, the first semiconductor layer 830,the activation layer 840, the second semiconductor layer 850, and thetransparent electrode 860 are sequentially formed on the substrate suchas sapphire substrate 810, and the conducting filaments 862 are formedin the transparent electrode 860 by performing the forming process onthe transparent electrode 860, so that the resistance state of thetransparent electrode 860 is changed into a low resistance state (referto (a) of FIG. 9).

At this time, the buffer layer 820 may be formed by using an un-dopedGaN layer; the first semiconductor layer 830 may be formed by using ann-AlGaN layer; and the activation layer 840 (MQW) is formed by usingAl(In)GaN/(In)GaN so as to generate light in an UV wavelength range; andthe second semiconductor layer 850 is formed by using a single layer ofp-AlGaN or a p-AlGaN layer and a p-GaN thin film.

As described above, the transparent electrode 860 formed on the secondsemiconductor layer 850 is formed by using the material which has hightransmittance with respect to light in an UV wavelength range and ofwhich resistance state is to be changed into a low resistance state dueto the conducting filaments 862 formed in the transparent electrode 860if a threshold voltage or more is applied to the transparent electrode860. Next, the forming process is performed by applying the thresholdvoltage or more, so that the conducting filaments 862 are formed in thetransparent electrode 860. Since the example of the resistance changematerial is described above, the detailed description thereof isomitted.

Next, the reflective layer 870 is formed on the transparent electrode860 by using a metal such as Ag or Al, and an bonding layer 880 forbonding to the submount substrate 880 is formed on the reflective layer870, so that a light emitting structure is completed (refer to (b) ofFIG. 9).

Next, the bonding layer 880 of the light emitting structure is bonded tothe submount substrate 880 so that the sapphire substrate 810 isdisposed at the upper side. In order to separate the sapphire substrate810 from the light emitting device, a UV laser beam having a wavelengthrange of 245 nm to 305 nm is irradiated through the sapphire substrate810. The irradiated UV laser beam is absorbed by the buffer layer 820,and the material GaN of the buffer layer 820 is decomposed into Ga andN₂, so that the sapphire substrate 810 is separated from the lightemitting device (refer to (c) of FIG. 9).

Next, the remaining material remaining on the first semiconductor layer830 is removed, and an n-type electrode pad is formed on the firstsemiconductor layer 830, so that the light emitting device is completed(refer to (d) of FIG. 9).

In the method of manufacturing the vertical type light emitting deviceaccording to the third embodiment, the processes except for the processof forming the transparent electrode 860 and the processes of performingthe forming process are the same as those of a general method ofmanufacturing a vertical type light emitting device, and thus, thedetailed description thereof is omitted.

On the other hand, similarly to the above-described first and secondembodiments, in the third embodiment, in order to prevent current frombeing concentrated on some area due to disconnection of some conductingfilaments 862 in the transparent electrode 860 to other conductingfilaments 862, in Step (b) of FIG. 9, a current spreading layer 990formed by using CNT or graphene may be further formed between thetransparent electrode 860 and the second semiconductor layer 850, or acurrent spreading layer formed by using CNT or graphene may be formed onthe transparent electrode 860, and the reflective layer 870 may beformed on the current spreading layer.

The modified example of the third embodiment of the present invention isillustrated in FIG. 8B. Referring to FIG. 8B, the current spreadinglayer 990 formed by using CNT or graphene is formed between the secondsemiconductor layer 850 and the reflective layer 870. The currentflowing through the reflective layer 870 is allowed to primarily spreadthrough the conducting filaments 862 in the transparent electrode 860and to spread from the current spreading layer which is in contact withthe transparent electrode 860 into the entire second semiconductor layer850, so that the current is uniformly injected.

On the other hand, it can be understood by the ordinarily skilled that,although the current spreading layer 990 is formed between thetransparent electrode 860 and the reflective layer 870 as describedabove, the same effect can be obtained.

In the light emitting device according to the third embodiment of thepresent invention described hereinbefore, the transparent electrode isformed by using a resistance change material having high transmittancewith respect to light in a UV wavelength range and good ohmic contactcharacteristic due to the conducting filaments, so that thetransmittance and electrical characteristics of the light emittingdevice can be improved.

FIGS. 10A to 10E are graphs illustrating a transmittance characteristic,an ohmic characteristic before the forming process, a contact resistancecharacteristic before the forming process, an ohmic characteristic afterthe forming process, and a contact resistance characteristic after theforming process of a transparent electrode in the case where thetransparent electrode is formed on a p-GaN semiconductor layer which isto be used as a second semiconductor layer of the above-described lightemitting device according to the present invention by using a Ga₂O₃material which is a resistance change material which is to be used toform the transparent electrode according to the present invention.

In the example illustrated in FIGS. 10A to 10E, a transparent electrode(thickness: about 80 nm) is formed on a p-GaN semiconductor layer byusing a Ga₂O₃ material.

Referring to the graph illustrated in FIG. 10A, in the example, it canbe understood that the Ga₂O₃ transparent electrode has transmittance of80% or more with respect to the light in a UV wavelength range of 264 nmor more. It can be understood that the transmittance of the transparentelectrode of the example is also greatly improved in comparison to thetransmittance of 20% of the ITO transparent electrode in the related artillustrated in FIG. 1.

FIGS. 10B to 10E illustrate ohmic characteristics (FIGS. 10B and 10D)and contact resistance characteristics (FIGS. 10C and 10E) measured byusing a TLM (Transfer Length Method) pattern in the case where adistance between measurement electrodes is 2 μm, 4 μm, 6 μm, 8 μm, and10 μm.

Referring to FIG. 10B, it can be understood that, in the case where thedistance between measurement electrodes is 2 μm, before the formingprocess, the current flowing into the transparent electrode is about1.0×10⁻¹¹ A irrespective of the applied voltage, so that the transparentelectrode does not have ohmic characteristic. In addition, referring toFIG. 10C, it can be understood that the ohmic contact resistancecharacteristic does not have linearity.

On the contrary, referring to FIG. 10D, it can be understood that, inthe case where the distance between measurement electrodes is 2 μm,after the forming process, when the voltage applied to the transparentelectrode is in a range of 0 V to 1.0 V, the current flowing into thetransparent electrode is about 0 A to 2.0×10⁻² A, which is 10⁹ times thecurrent flowing before the forming process. Accordingly, it can beunderstood that the ohmic characteristic is good so that the current isproportional to the voltage. In addition, referring to FIG. 10E, it canbe understood that the ohmic contact resistance characteristic haslinearity, and thus, the ohmic contact resistance characteristic isrelatively improved in comparison to the ohmic contact resistancecharacteristic before the forming process.

The characteristics of the Ga₂O₃ transparent electrode formed on thep-GaN semiconductor layer in the example illustrated in FIGS. 7A to 7Eare summarized as follows. The Ga₂O₃ transparent electrode hastransmittance of 80% or more with respect to the light in a UVwavelength range of 264 nm or more. In addition, as a result of themeasurement of the contact resistance by using the TLM pattern, thecontact resistance before the forming process is 51,680 Ωcm⁻², and thecontact resistance after the forming process is 2.64×10⁻⁵ Ωcm⁻².Therefore, the conductivity of the transparent electrode is greatlyimproved, and the ohmic characteristic thereof is good.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention as defined by the appended claims. The exemplary embodimentsshould be considered in descriptive sense only and not for purposes oflimitation. Therefore, the scope of the invention is defined not by thedetailed description of the invention but by the appended claims, andall differences within the scope will be construed as being included inthe present invention.

What is claimed is:
 1. A method of manufacturing a light emittingdevice, comprising: (a) sequentially forming a first semiconductorlayer, an activation layer which generates light, and a secondsemiconductor layer on a substrate; (b) forming a transparent electrodeon the second semiconductor layer by using a transparent insulatingmaterial of which resistance state is to be changed from a highresistance state into a low resistance state according to an appliedelectric field; and (c) changing the resistance state of the transparentelectrode into the low resistance state by applying a voltage to thetransparent electrode.
 2. The method according to claim 1, furthercomprising (d) forming a reflective layer on the transparent electrode.3. The method according to claim 2, further comprising: (e) etching thetransparent electrode, the second semiconductor layer, and theactivation layer so that the first semiconductor layer is exposed andforming an electrode pad on the first semiconductor layer; and (f)combining the substrate with a submount substrate so that the reflectivelayer is in contact with a first conductive pattern formed on thesubmount substrate and the electrode pad is in contact with the secondconductive pattern formed on the submount substrate through a bump. 4.The method according to claim 2, further comprising: (e) forming anbonding layer on the reflective layer and attaching a submount substrateto the bonding layer; and (f) separating the substrate from the firstsemiconductor layer.
 5. The method according to claim 1, wherein the (c)changing of the resistance state is performing a forming process byapplying a threshold voltage or more to the transparent electrode, sothat conducting filaments are formed in the transparent electrode. 6.The method according to claim 1, wherein the first semiconductor layeris formed with an n-AlGaN layer, and the second semiconductor layer isformed with a p-AlGaN layer or a p-AlGaN layer and a p-GaN thin film. 7.The method according to claim 1, wherein the transparent electrode is inohmic contact with the second semiconductor layer.
 8. The methodaccording to claim 1, further comprising, between the (a) sequentiallyforming of the first semiconductor layer, the activation layer whichgenerates light, and the second semiconductor layer and the (b) formingof the transparent electrode, forming a current spreading layer on thesecond semiconductor layer by using CNT or graphene, wherein, in the (b)forming of the transparent electrode, the transparent electrode isformed on the current spreading layer.
 9. The method according to claim1, further comprising forming a current spreading layer on thetransparent electrode of which resistance state is changed into the lowresistance state by using CNT or graphene.